In recent years, there have been an increasing number of occasions for ordinary households to receive directly a plurality of satellite broadcastings. This has occurred as satellite broadcastings have come to a stage of being wide spread, and communication satellite broadcastings have since started using commercial communication satellites. With this trend, there has been an increasing demand for stabilization of performance of microwave signaling equipment for reception and transmission.
An automatic microwave gain control device of the prior art is now described by referring to FIG. 6.
A microwave signal inputted through a microwave signal input terminal 68 is amplified by a microwave signal amplifier 61, and it is transferred to a microwave signal output terminal 60. A directional coupler 65 is connected at the microwave signal output terminal 60, and one end of the directional coupler 65 is connected to a microwave detection diode 66. A part of the microwave signal output is supplied to the diode 66 through the directional coupler 65. The diode 66 produces a D.C. voltage corresponding to the microwave signal output that, in turn, is supplied to a D.C. amplifier 67.
The D.C. voltage amplified by the D.C. amplifier 67 is supplied to a PIN diode 64, which functions as a variable attenuator. The PIN diode 64 connected to a tip of a half-wave stub 63 absorbs power of the microwave signal inputted from the microwave signal input terminal 68 according to a magnitude of the D.C. voltage. As a result, the automatic microwave gain control device of the prior art maintains a constant output power at the microwave signal output terminal 60 by this function of the PIN diode 64.
With this circuitry of the prior art, however, it is difficult to stabilize a detection characteristic at the super high frequency ("SHF") band, since the detection performance of the microwave detection diode 66 is affected by characteristic dispersion of the diode individually as well as dispersion of mounting position with the structure as described above. It has been a problem to reduce the characteristic dispersion of microwave detection diodes because the detection characteristic of especially the microwave detection diodes disperses more than the other circuit elements.